The use of endoscopes for diagnostic and therapeutic indications is rapidly expanding. There are now many types of specialized endoscopes, such as endoscopes for the upper esophagus, stomach, and duodenum; angioscopes for blood vessels; bronchoscopes for the bronchi; arthroscopes for joint spaces; colonscopes for the colon; and laparoscopes for the peritoneal cavity. The present invention applies to all types of endoscopes.
Typically, endoscopes have a long and flexible insertion tube with a diameter ranging between 15-25 millimeters. The insertion tube is inserted into a patient's body, along a selected path, during an endoscopic procedure. Multiple work channels usually extend along the length of the endoscope within the insertion tube. The work channels may allow inserting biopsy tools into and taking biopsies from the patient's body. Other mechanisms, which may be incorporated in the endoscope, are a visual imaging device, an illumination device, and a deflection mechanism. The proximal end of the endoscope usually has a handle in which the controls of the endoscope residue. Ordinarily, endoscopes are made of metallic, electrically conducting, materials. For example, U.S. Pat. No. 4,869,238 whose disclosure is incorporated herein by reference, describes a standard three-layer wall for endoscopes, containing metal coils and wire mesh.
Cleaning and sterilizing endoscopes are expensive and tedious procedures. Endoscopes incorporate expensive and delicate apparatus which may be damaged during cleaning. Also, the long and narrow work channels in the insertion tube are difficult to clean.
Disposable endoscopic sheaths have been developed, to avoid the need for cleaning and sterilizing endoscopes. These sheaths substantially isolate the endoscope from the patient, and thus prevent the endoscope from being contaminated. Some of these sheaths have thick walls containing work channels within them, leaving only part of their cross-section for a lumen which receives the insertion tube of the endoscope. The walls of the work channels and the area between the work channels usually comprise the same material as the outer wall.
A sheath with thick walls is described, for example, in PCT publication WO 94/287282, whose disclosure is incorporated herein by reference. WO 94/28782 describes a disposable sheath which may include work channels. The sheath removably receives a cylindrical insertion tube which contains controls and other delicate apparatus of the endoscope. Another disposable sheath is described in U.S. Pat. No. 5,483,951, whose disclosure is incorporated herein by reference. This disposable sheath comprises a thin outer wall, inner work channels, and a lumen with a "D" shaped cross-section. The lumen is adapted to receive and substantially isolate a non-disposable insertion tube of an endoscope, which is accordingly "D" shaped.
many endoscopic procedures involve irreversible actions such as taking tissue samples and ablation at the distal end of the insertion tube of the endoscope. Performing these actions at an incorrect position can damage important blood vessels or nerves, puncture the intestine, or otherwise cause severe damage to the patient. Therefore it is useful to have a method of determining the position and/or orientation of the distal end of the endoscope.
Through a visual imaging device the user can observe images transmitted from the distal end of the endoscope. From these images and from knowledge of the path the endoscope has followed, the user can ordinarily determined the position of the endoscope. However, there are organs of the human body in which the images and knowledge of the path do not suffice to determine the position of the endoscope to sufficient accuracy. Some organs, such as the brain, have a homogeneous appearance in which it is very hard or even impossible to find a specific point based only on the images from the imaging device. In addition, determining the position of he endoscope from the images could be very time consuming. In many endoscopic procedures, such as endoscopic bypass surgery, the amount of the time a patient can endure the endoscopic procedure is limited.
In some procedures, the endoscope is used to map a section of an organ. The map is produced by systematically bringing the distal end of the endoscope in contact with a plurality of points within the organ and registering the positions of the points. To confirm that the entire section of the organ has been mapped, a sufficient density of points must be registered within the section. To insure use of a sufficient density of points it is necessary to have unique position identification for every point.
Another problem which arises, for example, in colonscope procedures, is formation of loops in the long and narrow tube of the colonscope. Such loops may arise when the insertion tube encounters an obstacle, or gets stuck in a narrow passage. Instead of progressing, the tube forms loops within the patient. In an attempt to proceed in insertion of the colonscope, excess force may be exerted, damaging delicate tissue in the patient's body. The user may proceed with the attempted insertion of the endoscope without realizing there is a problem. The ability to see the configuration of the endoscopic insertion tube within the patient's body, allows early discovery of the existence of loops and makes straightening them simpler.
One method used in the art of determine the configuration of the insertion tube is x-ray imaging. Another method used is magnetic field positioning, which avoids the x-ray exposure to the patient and the operator. PCT application PCT/GB93/01736, whose disclosure is incorporated herein by reference, describes a method of magnetic filed position determination using low frequency magnetic fields to determine the position of a miniature sensor embedded within a colonscope tube. Based on the position of the sensor at sequential time periods, an image of the configuration of the colonscope tube is produced.
In tests mentioned in PCT/GB93/01736 it was found that there were some distortions in the image due to the metallic construction of the colonscope. The metallic construction of the colonscope reacts with the sensing magnetic filed in that currents are induced in the colonscope by the magnetic filed. These currents, called eddy currents, generate a disturbing magnetic field which is overlaid on the sensing magnetic field. Thus, the amplitude and/or phase of the magnetic field used by the position determining system are changed in proximity of metallic substances. The magnitude and effect of the eddy currents depend on the size and geometry of the metallic substance. For example, large metal rings change the magnetic field substantially in their proximity. Conversely, small metal objects and objects with a relatively high resistance, within which substantially no eddy currents are formed, does not substantially affect the magnetic field.
Magnetic filed position determining systems typically determine positions according to the magnetic filed's amplitude and/or its phase. Changes in the amplitude and/or phase due to eddy currents cause inaccuracies in determined positions and interfere with precise determination of positions. Interference can also arise from ferro-magnetic materials in the endoscope, which concentrate the magnetic filed in their proximity. Thus, ferro-magnetic materials cause distortions in the magnetic field, changing the amplitude and phase of the field at measured points.
The interference is dependent on the frequency of a drive signal which generates the magnetic field. A high drive signal frequency is preferred in order to enhance sensor sensitivity, but must be limited so as not to intensify the interference to the position determining system. Therefore, the PCT/GB93/10736 system makes a compromise in its choice of the frequency used. If a different method to minimize the interference is used, it would be possible to enjoy the advantages of a higher drive signal frequency.
Existing catheters have a metal coil (for structural purposes) within them. The coil extends along the length of the catheter except for a small part of the distal end of the catheter. A sensor coupled with a magnetic filed position determining system is embedded within the distal end of the catheter.